Sealing plate for prismatic secondary battery and prismatic secondary battery using the sealing plate

ABSTRACT

A prismatic secondary battery includes a prismatic hollow outer body having a mouth and a bottom; a flat electrode assembly, a positive electrode collector, a negative electrode collector, and an electrolyte, all of which are stored in the prismatic outer body; a sealing plate sealing up the mouth of the prismatic outer body; and a positive electrode terminal attached to the sealing plate in an electrically insulated manner. The sealing plate includes a gas release valve and an electrolyte pour hole and further includes, on the front face, a concaved flat face having an identification code. With the prismatic secondary battery of the invention, a jig for assembly or the like is unlikely to come into contact with the identification code during an assembly process of the prismatic secondary battery, hence the identification code is unlikely to be abraded, and the traceability is unlikely to be lost.

TECHNICAL FIELD

The present invention relates to a sealing plate for a prismaticsecondary battery and a prismatic secondary battery using the sealingplate.

BACKGROUND ART

Alkaline secondary batteries typified by a nickel-hydrogen battery andnonaqueous electrolyte secondary batteries typified by a lithium ionbattery are widely used as power supplies for driving portableelectronic equipment such as cell phones including smartphones, portablecomputers, PDAs, and portable music players. In addition, alkalinesecondary batteries and the nonaqueous electrolyte secondary batteriesare also widely used for power supplies for driving electric vehicles(EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationarystorage battery systems for suppressing the variation in output power ofphotovoltaic generation, wind power generation, and the like, and forpeak shifts in system power in order to store electric power during thenight time and to use the electric power during daytime.

In particular, the batteries for EVs, HEVs, and PHEVs and for thestationary storage battery system are required to have high capacity andhigh output characteristics, and hence each battery is upsized and anumber of batteries are connected in series or parallel when used. Toaddress this, in these applications, prismatic secondary batteries aregenerally used from the viewpoint of space efficiency. A prismaticsecondary battery that further needs physical strength commonly employs,as an outer body of the battery, a metal prismatic outer body having amouth and a metal sealing plate for sealing up the mouth.

Such a prismatic secondary battery, for example, a prismatic nonaqueouselectrolyte secondary battery, is produced as follows. For example, bothfaces of a positive electrode substrate made from, for example, a longsheet of aluminum foil, are coated with a positive electrode activematerial mixture containing a positive electrode active material toprepare a positive electrode sheet. Separately, both faces of a negativeelectrode substrate made from, for example, a long sheet of copper foil,are coated with a negative electrode active material mixture containinga negative electrode active material to prepare a negative electrodesheet.

Next, the positive electrode sheet and the negative electrode sheet arestacked interposing a separator made from, for example, a microporouspolyethylene film therebetween, and the positive electrode sheet and thenegative electrode sheet are spirally wound on a cylindrical windingcore while insulating the positive electrode sheet and the negativeelectrode sheet from each other through the separator to prepare acylindrical wound electrode assembly. Then, the cylindrical woundelectrode assembly is pressed with a pressing machine to form a flatwound electrode assembly. Next, a positive electrode collectorelectrically connected to the positive electrode sheet is electricallyconnected to a positive electrode terminal that is insulated from asealing plate, while a negative electrode collector electricallyconnected to the negative electrode sheet is electrically connected to anegative electrode terminal that is insulated from a sealing plate.Then, the flat wound electrode assembly is wrapped with a member havinginsulating characteristics and stored in a metal prismatic outer body; amouth portion of the prismatic outer body is sealed with a sealingplate; an electrolyte is poured from a electrolyte pour hole provided onthe sealing plate; and finally the electrolyte pour hole is sealed toproduce the prismatic nonaqueous electrolyte secondary battery.

Such a prismatic secondary battery required to have high capacity andhigh output characteristics is required to have much higher safety thanthat of secondary batteries for portable small equipment. Especially, inthe case of a nonaqueous electrolyte secondary battery that uses amaterial having very high reactivity, for example, as shown in US PatentPublication No. 2010/0233529 (US2010/0233529 (A1)) and U.S. Pat. No.7,781,088 specification (U.S. Pat. No. 7,781,088 (B2)), this nonaqueouselectrolyte secondary battery is equipped with a gas release valve forreleasing internal pressure when the pressure in a battery outer body isincreased and a current interruption mechanism for interruptingelectrical connection between an external terminal and an electrodeassembly in the outer body.

The metal sealing plate used for the prismatic secondary batteryincludes at least a mouth for attaching a positive electrode terminal, amouth for attaching a negative electrode terminal, a gas release valve,and an electrolyte pour hole. The metal sealing plate commonly has arectangular shape, a chamfered rectangular shape, a rounded rectangularshape, or an oval shape. The mouth for attaching a positive electrodeterminal and the mouth for attaching a negative electrode terminal arearranged on both end sides in a longitudinal direction of the sealingplate, and each of the gas release valve and the electrolyte pour holeis provided between the negative electrode terminal and the positiveelectrode terminal on the sealing plate.

Meanwhile, the prismatic secondary battery is mass-produced and thus ispreferred to have a sealing plate with any identification code forproviding traceability during an assembly process and after theassembly. Such an identification code can be easily formed by printing,laser marking, or seal-affixing on the sealing plate. However, thesealing plate is required to be subjected to an attachment process of apositive electrode terminal and a negative electrode terminal, anattachment process of a negative electrode collector and a positiveelectrode to the positive electrode terminal and the negative electrodeterminal, respectively, an attachment process of the collectors to aprismatic outer body, a pouring process of an electrolyte, and the likeuntil the completion of a prismatic secondary battery. Due to theseprocesses, even when an identification code is simply applied byprinting, laser marking, seal-affixing, or the like onto a surface ofthe sealing plate, such an identification code is abraded by the contactof a jig for assembly or the like during the assembly process of theprismatic secondary battery, and this increases the possibility that theidentification code cannot be read thereby causing traceability to belost.

SUMMARY

An advantage of some aspects of the present invention is to provide, byachieving a structure in which a jig for assembly or the like isunlikely to come into contact with an identification code during anassembly process of a prismatic secondary battery, a sealing plate for aprismatic secondary battery having the structure by which theidentification code is unlikely to be abraded and the traceability isunlikely to be lost and a prismatic secondary battery using the sealingplate.

According to an aspect of the invention, a sealing plate for a prismaticsecondary battery includes a pair of mouths for attaching a positiveelectrode terminal and a negative electrode terminal, one mouth beingformed on one end side in a longitudinal direction of the sealing plate,and the other mouth being formed on the other end side, and a gasrelease valve and an electrolyte pour hole provided between the pair ofmouths. In the sealing plate for a prismatic secondary battery, thesealing plate includes a front face having a concaved flat face, and anidentification code formed on the concaved flat face.

The sealing plate for a prismatic secondary battery of the inventionincludes the concaved flat face that is formed on the front face of thesealing plate and that has the flat face having a height less than thatof the peripheral portions of the sealing plate and includes theidentification code formed on the concaved flat face. Hence, even when ajig for assembling the prismatic secondary battery or the like comesinto contact with the sealing plate, such a jig is unlikely to come intocontact with the identification code part. On this account, with thesealing plate for a prismatic secondary battery of the invention, theidentification code is unlikely to be abraded, and therefore thetraceability is unlikely to be lost during an assembly process and afterthe assembly of the prismatic secondary battery. The sealing plateusable in the invention may have, for example, a rectangular shape, achamfered rectangular shape, a rounded rectangular shape, or an ovalshape in a planar view.

The “front face” of the sealing plate for a prismatic secondary batteryof the invention represents a portion positioned on the outer side ofthe prismatic secondary battery, while a portion of the sealing platepositioned on the inner side of the prismatic secondary battery isexpressed as a “back face”. The “concaved flat face” in the sealingplate for a prismatic secondary battery of the invention may include aso-called “groove” including a flat face that has a height less thanthat of the other portions and that is formed across the whole width ofthe sealing plate, however, preferably includes a flat face having aheight less than that of the peripheral portions of the sealing plate.On this account, with the sealing plate for a prismatic secondarybattery of the invention, the reduction in flexural strength withrespect to the longitudinal direction of the sealing plate is small inthe concaved portion. This can suppress the deformation of the sealingplate when the internal pressure of the prismatic secondary battery isincreased and can stabilize working pressure of the gas release valve.Moreover, the sealing plate for a prismatic secondary battery of theinvention does not generate a level difference in the fitting portionbetween the sealing plate and the battery outer body, and this enablesuniform laser-welding of the fitting portion between the sealing plateand the battery outer body.

In the sealing plate for a prismatic secondary battery of the invention,an applying mode of the identification code may be any known mode, forexample, printing, laser marking, and seal-affixing. Among them, lasermarking is preferably adopted because the identification code applied isunlikely to be abraded or peeled, and is unlikely to be misread.Examples of the usable types of identification codes include awell-known code including a simple code such as numbers, characters, andsymbols, a one-dimensional code such as a bar-code, and atwo-dimensional code such as a stacked bar-code. Among them, thetwo-dimensional code is desirable because it can contain a large amountof information and is misread less frequently even when the code hasbeen abraded or peeled away.

In the sealing plate for a prismatic secondary battery, it is preferablethat the sealing plate include a back face having a convex portion at aposition corresponding to the concaved flat face.

When the concaved flat face is formed by forging, the convex portion canbe easily formed on the back face of the sealing plate at a positioncorresponding to the concaved flat face. In addition, the presence ofthe concaved portion on the front face and the convex portion on theback face increases the flexural strength of the sealing plate in boththe longitudinal direction and the width direction. Hence, such astructure can further suppress the deformation of the sealing plate whenthe internal pressure of the prismatic secondary battery is increasedand can further stabilize working pressure of the gas exhaust valve gasrelease valve.

In the sealing plate for a prismatic secondary battery according to theaspect, an additional identification code may be applied on at least oneside adjacent to the convex portion in the longitudinal direction of thesealing plate.

In the longitudinal direction of the sealing plate, both sides adjacentto the convex portion are positions at which a jig for assembling theprismatic secondary battery or the like is unlikely to physically comeinto contact. The back face of the sealing plate is positioned inside ofthe prismatic secondary battery and is not exposed to the outside afterthe completion of the prismatic secondary battery. Thus, with thesealing plate for a prismatic secondary battery of the invention, theadditional identification code is unlikely to be abraded and hence thetraceability is unlikely to be lost, at least during the assemblyprocess of the prismatic secondary battery.

The additional identification code may be the same as or different fromthe identification code applied onto the concaved flat face. However,the additional identification code is not exposed to the outside afterthe completion of the prismatic secondary battery and is entirely usedto ensure the traceability during an assembly process. Thus, theadditional identification code is preferably different from theidentification code applied onto the concaved flat face.

In the sealing plate for a prismatic secondary battery according to theaspect, it is desirable that the convex portion be provided at aposition adjacent to the gas release valve and that the additionalidentification code be formed at a position opposite to the gas releasevalve with respect to the convex portion.

The presence of the convex portion at a position adjacent to the gasrelease valve further increases the flexural strength of the sealingplate in both the longitudinal direction and the width direction. Hence,such a structure can further suppress the deformation of the sealingplate when the internal pressure of the prismatic secondary battery isincreased and can further stabilize working pressure of the gas releasevalve.

According to another aspect of the invention, a prismatic secondarybattery includes: a prismatic hollow outer body having a mouth and abottom; a flat electrode assembly including a positive electrode sheetand a negative electrode sheet, a positive electrode collectorelectrically connected to the positive electrode sheet, a negativeelectrode collector electrically connected to the negative electrodesheet, and an electrolyte, all being stored in the prismatic outer body;a sealing plate sealing up the mouth of the prismatic outer body; and apositive electrode terminal and a negative electrode terminal attachedto the sealing plate in a manner electrically insulated from the sealingplate, while electrically connected to the positive electrode collectorand the negative electrode collector, respectively. In the prismaticsecondary battery, on the sealing plate, the positive electrode terminalis fixed on one end side in a longitudinal direction of the sealingplate and the negative electrode terminal is fixed on the other endside, a gas release valve and an electrolyte pour hole are formedbetween the positive electrode terminal and the negative electrodeterminal, the sealing plate includes a front face having a concaved flatface, and an identification code formed on the concaved flat face.

In the prismatic secondary battery of the invention, the concaved flatface is formed on the front face of the sealing plate and has a flatface having a height less than that of the peripheral portions of thesealing plate, and the identification code is formed on the concavedflat face. Hence, even when a jig for assembling the prismatic secondarybattery or the like comes into contact with the sealing plate, such ajig is unlikely to come into contact with the identification code part.On this account, with the prismatic secondary battery of the aspect, theidentification code is unlikely to be abraded, and therefore thetraceability is unlikely to be lost after the assembly of the prismaticsecondary battery. The sealing plate usable in the prismatic secondarybattery may be a metal plate having, for example, a rectangular shape, achamfered rectangular shape, a rounded rectangular shape, or an ovalshape.

Moreover, with the prismatic secondary battery of the aspect, thereduction in flexural strength with respect to the longitudinaldirection of the sealing plate is small in the concaved portion, andthis can suppress the deformation of the sealing plate when the internalpressure of the prismatic secondary battery is increased and canstabilize working pressure of the gas release valve. Furthermore, theprismatic secondary battery of the aspect does not generate a leveldifference in the fitting portion between the sealing plate and thebattery outer body, and this enables uniform laser-welding of thefitting portion between the sealing plate and the battery outer body. Inaddition, when the concaved flat face is formed by forging, a convexportion is formed on the back face. The concaved portion increases theflexural strength of the sealing plate in the longitudinal direction,and this can further stabilize working pressure of the gas releasevalve.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1A is a front view of a prismatic nonaqueous electrolyte secondarybattery of an embodiment and FIG. 1B is a plan view of the prismaticnonaqueous electrolyte secondary battery.

FIG. 2A is a partial cross-sectional view taken along the line IIA-IIAin FIG. 1B and FIG. 2B is a partial cross-sectional view taken along theline IIB-IIB in FIG. 2A.

FIG. 3A is a plan view of a sealing plate of the embodiment, FIG. 3B isa cross-sectional view taken along the line IIIB-IIIB in FIG. 3A, andFIG. 3C is a bottom view of the sealing plate of the embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENT

Hereinafter, embodiment of the invention will be described in detailwith reference to the accompanying drawings. However, the embodimentdescribed below is intended to exemplify the technical spirit of theinvention, the invention is not intended to be limited to theembodiment, and the invention may equally be applied to various modifiedcases without departing from the technical spirit described in theclaims. In each drawing used for explanation in the specification, eachmember is appropriately shown on a different scale so that the memberhas a recognizable size in each drawing and the members are notnecessarily shown in proportion to the actual sizes.

Embodiment

Firstly, as an example of a prismatic secondary battery of theembodiment, a prismatic nonaqueous electrolyte secondary battery will bedescribed with reference to FIG. 1 and FIG. 2. FIG. 1A is a front viewof the prismatic nonaqueous electrolyte secondary battery, and FIG. 1Bis a plan view of the prismatic nonaqueous electrolyte secondarybattery. FIG. 2A is a cross-sectional view taken along the line IIA-IIAin FIG. 1B, and FIG. 2B is a cross-sectional view taken along the lineIIB-IIB in FIG. 2A.

A prismatic nonaqueous electrolyte secondary battery 10 includes a flatwound electrode assembly 11 in which a positive electrode sheet and anegative electrode sheet are wound while being insulated from each otherthrough a separator, which are not shown in the drawings. The positiveelectrode sheet is prepared by coating both faces of a positiveelectrode substrate made from aluminum foil with a positive electrodeactive material mixture, then drying and rolling the coated substrate,and slitting the substrate so as to expose the aluminum foil in a stripshape. Separately, the negative electrode sheet is prepared by coatingboth faces of a negative electrode substrate made from copper foil witha negative electrode active material mixture, then drying and rollingthe coated substrate, and slitting the substrate so as to expose thecopper foil in a strip shape.

Then, the flat wound electrode assembly 11 is prepared as follows. Thepositive electrode sheet and the negative electrode sheet obtained asabove are stacked interposing a polyolefin microporous separatortherebetween so as to displace the aluminum foil exposed portion of thepositive electrode sheet and the copper foil exposed portion of thenegative electrode sheet from the corresponding counter electrode activematerial mixtures, and the whole is wound while insulating the positiveelectrode sheet and the negative electrode sheet from each other throughthe separator to afford the flat wound electrode assembly 11 includingone end with a plurality of stacked positive electrode substrate exposedportions 12 and the other end with a plurality of stacked negativeelectrode substrate exposed portions 13.

The plurality of stacked positive electrode substrate exposed portions12 are electrically connected through a positive electrode collector 14made of aluminum to a positive electrode terminal 15 also made ofaluminum, and similarly, the plurality of stacked negative electrodesubstrate exposed portions 13 are electrically connected through anegative electrode collector 16 made of copper to a negative electrodeterminal 17 also made of copper. The positive electrode terminal 15 andthe negative electrode terminal 17 are, as shown in FIG. 2A and FIG. 2B,fixed to a sealing plate 20 made of, for example, aluminum through aninsulating members 18 and 19, respectively. The positive electrodeterminal 15 and the negative electrode terminal 17 are, as necessary,connected to an external positive electrode terminal and an externalnegative electrode terminal (not shown in the drawings) arranged on theinsulating members 18 and 19, respectively.

The flat wound electrode assembly 11 prepared as above is wrapped with aresin sheet (not shown in the drawings) having insulatingcharacteristics except the sealing plate 20 side and is inserted into aprismatic outer body 21 made of, for example, aluminum having an openface. Then, the sealing plate 20 is fitted to the mouth portion of theprismatic outer body 21; a fitting portion 22 between the sealing plate20 and the outer body 21 is laser-welded; then, a nonaqueous electrolyteis poured from an electrolyte pour hole 23; and the electrolyte pourhole 23 is sealed up to produce the prismatic nonaqueous electrolytesecondary battery 10 of the embodiment.

A current interruption mechanism 24 that works correspondingly to thepressure of gas generated in the battery is provided between thepositive electrode collector 14 and the positive electrode terminal 15.The sealing plate 20 also equips a gas release valve 25 that opens whenthe gas pressure becomes higher than the working pressure of the currentinterruption mechanism 24. Thus, the inside of the prismatic nonaqueouselectrolyte secondary battery 10 is hermetically sealed. The prismaticnonaqueous electrolyte secondary battery 10 is used alone or used with aplurality of the batteries connected in series or parallel, for variousapplications. When a plurality of the prismatic nonaqueous electrolytesecondary batteries 10 are connected in series or parallel to be used,an external positive electrode terminal and an external negativeelectrode terminal are preferably provided separately for connectingeach battery through bus bars so as to send a large current.

In the flat wound electrode assembly 11, as shown in FIG. 2A and FIG.2B, on the positive electrode sheet side, the plurality of stackedpositive electrode substrate exposed portions 12 are divided into twoportions, and between the portions, a positive electrode intermediatemember 27 made of resin and having a plurality of positive electrodeconnection conductive members 26, two positive electrode connectionconductive members 26 in the embodiment, is interposed. In a similarmanner, on the negative electrode sheet side, the plurality of stackednegative electrode substrate exposed portions 13 are divided into twoportions, and between the portions, a negative electrode intermediatemember 29 made of resin and having two negative electrode connectionconductive members 28 is interposed. On the outermost surface of thepositive electrode substrate exposed portions 12 positioned on each sideof the positive electrode connection conductive member 26, a positiveelectrode collector 14 is disposed, and on the outermost surface of thenegative electrode substrate exposed portions 13 positioned on each sideof the negative electrode connection conductive member 28, a negativeelectrode collector 16 is disposed. The positive electrode connectionconductive member 26 is made of aluminum that is the same material asthe positive electrode substrate, the negative electrode connectionconductive member 28 is made of copper that is the same material as thenegative electrode substrate, and the shape of the positive electrodeconnection conductive member 26 may be the same as or different fromthat of the negative electrode connection conductive member 28.

The positive electrode collector 14 is resistance-welded to the positiveelectrode substrate exposed portion 12 (at four points), and thepositive electrode substrate exposed portion 12 is resistance-welded tothe positive electrode connection conductive member 26 (at four points),for connection. Similarly, the negative electrode collector 16 isresistance-welded to the negative electrode substrate exposed portion 13(at four points), and the negative electrode substrate exposed portion13 is resistance-welded to the negative electrode connection conductivemember 28 (at four points), for connection. FIG. 2A shows, on thepositive electrode side, two weld marks 30 formed by the resistancewelding and, on the negative electrode side, two weld marks 31.

Hereinafter, in the flat wound electrode assembly 11 of the Embodiment,the resistance welding method using the positive electrode substrateexposed portions 12, the positive electrode collector 14, and thepositive electrode intermediate member 27 having the positive electrodeconnection conductive members 26 and the resistance welding method usingthe negative electrode substrate exposed portions 13, the negativeelectrode collector 16, and the negative electrode intermediate member29 having the negative electrode connection conductive members 28 willbe described in detail. However, in the Embodiment, the shapes of thepositive electrode connection conductive member 26 and the positiveelectrode intermediate member 27 may be substantially the same as theshapes of the negative electrode connection conductive member 28 and thenegative electrode intermediate member 29, and each resistance weldingmethod may be substantially the same. Therefore, the method for thepositive electrode sheet side will be described below as a typicalexample.

First, the positive electrode substrate exposed portions 12 of the flatwound electrode assembly 11 prepared as above were divided from thewound center to both side into two portions, and the divided positiveelectrode substrate exposed portions 12 were gathered to a center as aquarter of the thickness of the electrode assembly. Then, the positiveelectrode collectors 14 were disposed on both sides of the outermostperiphery of the positive electrode substrate exposed portions 12, thepositive electrode intermediate member 27 having the positive electrodeconnection conductive members 26 was inserted between both sides of thebisectional positive electrode substrate exposed portions 12 so thatprotrusions on both sides of the positive electrode connectionconductive member 26 were in contact with the positive electrodesubstrate exposed portions 12 on the inner periphery thereof. Thepositive electrode collector 14 is made from, for example, an aluminumplate having a thickness of 0.8 mm.

Here, the positive electrode connection conductive member 26 held withthe positive electrode intermediate member 27 in the Embodiment has acolumn-shaped body with two opposed faces, and a protrusion (projection)having, for example, a truncated cone shape is formed on each face. Thepositive electrode connection conductive member 26 may have any shape,for example, a prismatic shape and an elliptical column shape, inaddition to the cylindrical shape, as long as it is a metal block.Examples of the material usable for forming the positive electrodeconnection conductive member 26 include copper, a copper alloy,aluminum, an aluminum alloy, tungsten, and molybdenum. In addition tothe members made of these metals, for example, a member having aprotrusion coated with nickel or a member in which the material of aprotrusion and the vicinity of a bottom of the protrusion are changedinto a metal capable of accelerating heat generation, such as tungstenand molybdenum and such a protrusion is bonded to the cylindrical-shapedbody of the positive electrode connection conductive member 26 made ofcopper, a copper alloy, aluminum, or an aluminum alloy by brazing or thelike may be used.

A plurality of the positive electrode connection conductive members 26of the Embodiment, for example, two positive electrode connectionconductive members are integrally held with the positive electrodeintermediate member 27 made of a resin material. In this case, thepositive electrode connection conductive members 26 are held so as to beparallel to each other. The positive electrode intermediate member 27may have any shape, for example, a prismatic shape and a column shape.However, the shape is preferably a prismatic shape having a long laterallength in order to be fixed between the bisectional positive electrodesubstrate exposed portions 12 while being stably positioned. However,corners of the positive electrode intermediate member 27 are preferablychamfered so as not to scratch or deform the soft positive electrodesubstrate exposed portion 12 even when the corner comes into contactwith the positive electrode substrate exposed portion 12. At least apart to be inserted between the bisectional positive electrode substrateexposed portions 12 may be chamfered.

The length of the prismatic positive electrode intermediate member 27varies depending on the size of the prismatic nonaqueous electrolytesecondary battery 10, but may be 20 mm to several tens of mm. The widthof the prismatic positive electrode intermediate member 27 may besubstantially the same as the height of the positive electrodeconnection conductive member 26, but the prismatic positive electrodeintermediate member 27 may have a width of which at least both ends ofthe positive electrode connection conductive member 26 to be welded areexposed. Both ends of the positive electrode connection conductivemember 26 preferably protrude from the surface of the positive electrodeintermediate member 27, but may not necessarily protrude. With such astructure, the positive electrode connection conductive member 26 isheld with the positive electrode intermediate member 27 and the positiveelectrode intermediate member 27 is disposed between the bisectionalpositive electrode substrate exposed portions 12 while being stablypositioned.

Next, between a pair of resistance welding electrode rods (not shown inthe drawings), the positive electrode collector 14 and the flat woundelectrode assembly 11 with the positive electrode intermediate member 27holding the positive electrode connection conductive members 26 aredisposed. The pair of the resistance welding electrode rods are broughtinto contact with the positive electrode collectors 14 disposed on bothoutermost periphery sides of the positive electrode substrate exposedportions 12. Then, an appropriate pressure is applied between the pairof the resistance welding electrode rods to perform resistance weldingin a previously determined constant condition. During the resistancewelding, the positive electrode intermediate member 27 is disposed whilebeing stably positioned between the bisectional positive electrodesubstrate exposed portions 14. This improves the dimensional precisionbetween the positive electrode connection conductive member 26 and thepair of electrode rods, enables the resistance welding in an exact andstable condition, and can suppress variation in the welding strength.

Next, the specific structure of the sealing plate of the invention willbe described with reference to FIG. 3. FIG. 3A is a plan view of asealing plate of the embodiment, FIG. 3B is a cross-sectional view takenalong the line IIIB-IIIB in FIG. 3A, and FIG. 3C is a bottom view of thesealing plate of the embodiment.

The sealing plate 20 has, for example, a rectangular shape so as to befitted onto the mouth portion of the prismatic outer body 21 to sealelectrolyte therein. The sealing plate 20 has a main surface which isexposed to the outward when the sealing plate 20 is fitted onto theprismatic outer body 21. The main surface is provided with a positiveelectrode terminal 15 and a negative electrode terminal 17. Moreprecisely, The main surface of the sealing plate 20 includes a mouth 34for attaching the positive electrode terminal 15 and a mouth 35 forattaching the negative electrode terminal 17 on each end side, andincludes an electrolyte pour hole 23, a gas release valve 25, and aconcaved flat face 32 between the mouths 34 and 35. The concaved flatface 32 has a flat face having a height less than that of the peripheralportions in the sealing plate 20 and being partially formed in a widthdirection (short side direction) of the sealing plate 20. That is tosay, the main surface of the sealing plate 20 is formed with a step sothat the concaved flat face is formed at the height less than that ofthe main surface of the sealing plate 20. When the concave portion 32 isformed across the whole width of the sealing plate 20, it becomes aso-called “groove.” Such a groove largely reduces the flexural strengthof the sealing plate 20 in the longitudinal direction as well asgenerating a level difference between the sealing plate 20 and the endface of the outer body 21 thereby to interfere with uniformlaser-welding.

The sealing plate 20 of the embodiment further includes anti-rotationprotrusions 38 and 40 and auxiliary protrusions 39 and 41 on both sidesof the mouth 34 for the positive electrode and the mouth 35 for thenegative electrode, respectively, in order to attach the externalpositive electrode terminal and the external negative electrode terminal(not shown in the drawings) used when a plurality of the prismaticnonaqueous electrolyte secondary batteries are connected in series orparallel. The anti-rotation protrusions 38 and 40 and the auxiliaryprotrusions 39 and 41 are fitted to the external positive electrodeterminal and the electrode negative external terminal that are providedas necessary, and function so that the external positive electrodeterminal and the external negative electrode terminal are unlikely torotate.

For the sealing plate 20 of the embodiment, a rectangular aluminum plateis punched out to form the mouths 34 and 35 and the electrolyte pourhole 23 by forging, and the gas release valve 25, the concaved flat face32, the anti-rotation protrusions 38 and 40, and the auxiliaryprotrusions 39 and 41 are simultaneously formed. In the sealing plate20, concaves 38 a, 40 a, 39 a, and 41 a formed on the front face aroundthe anti-rotation protrusions 38 and 40 and the auxiliary protrusions 39and 41 as well as concaves 38 b, 40 b, 39 b, and 41 b formed in the backface are formed by forging so that each of the anti-rotation protrusions38 and 40 and the auxiliary protrusions 39 and 41 has a height largerthan that of the front face of the sealing plate 20. A convex portion 36is formed on the back face of the sealing plate 20 corresponding to theconcaved flat face 32 formed position.

The formation of the convex portion 36 on the back face of the sealingplate 20 corresponding to the concaved flat face 32 formed position asabove increases the flexural strength of the sealing plate 20 in boththe longitudinal direction and the width direction, compared with thecase without the convex portion 36. Therefore, a prismatic nonaqueouselectrolyte secondary battery 10 produced using the sealing plate 20suppresses the deformation of the sealing plate 20 when the internalpressure is increased, and this can stabilize working pressure of thegas release valve 25.

In the sealing plate 20 of the embodiment, a two-dimensional code as afirst identification code 33 is formed on the flat face as the bottom ofthe concaved flat face 32 on the front face by, for example, lasermarking, while, a second identification code 37 is similarly formed bylaser marking on a surface of a side adjacent to the convex portion 36on the back face.

The first identification code 33 is formed on the concaved flat facehaving a height less than that of the peripheral portions that areformed on the front face of the sealing plate 20. Thus, even when a jigfor assembling the prismatic nonaqueous electrolyte secondary battery 10or the like comes into contact with the sealing plate 20, such a jig isunlikely to physically come into contact with the first identificationcode 33. Therefore, the first identification code 33 is unlikely to beabraded, and the traceability is unlikely to be lost during the assemblyprocess and after the assembly of the prismatic nonaqueous electrolytesecondary battery 10. The second identification code 36 is not exposedto the outside after the assembly of the prismatic nonaqueouselectrolyte secondary battery 10, but is used for tracing during anassembly process of the prismatic nonaqueous electrolyte secondarybattery 10. The first identification code 33 may be the same as ordifferent from the second identification code 36.

Here, the laser marking is exemplified for explaining the application ofthe first identification code 33 and the second identification code 36,but well-known printing or seal-affixing may be adopted. However, thelaser marking is preferably adopted because an applied code is unlikelyto be abraded or peeled and is unlikely to be misread. Examples of theusable type of the identification code includes, in addition to thetwo-dimensional code, a simple code such as numbers, characters, andsymbols, a one-dimensional code such as a bar-code. Among them, thetwo-dimensional code is desirable because it can contain a large amountof information and is misread less frequently even when the code hasbeen abraded or peeled away.

As the sealing plate 20 of the embodiment, the exemplified sealing platehas the convex portion 36 on the back face corresponding to the concavedflat face 32 that is formed on the front face, but the convex portion 36is not a necessary component. For example, when the concaved flat face32 is formed on the front face by cutting work, the convex portion isnot formed. However, a sealing plate without such a convex portion doesnot have the advantage of abrasion resistance or peel resistance withrespect to the identification code formed on the back face as well ashaving a smaller flexural strength in the longitudinal direction of thesealing plate 20. To address this, the convex portion 36 is preferablyformed on the back face corresponding to the concaved flat face 32 thatis formed on the front face as in the case of the sealing plate of theembodiment.

The embodiment describes the prismatic nonaqueous electrolyte secondarybattery as an example of the prismatic secondary battery. However, theinvention is also applicable to a prismatic secondary battery using anaqueous electrolyte, such as a nickel-hydrogen secondary battery.

1. A sealing plate for a prismatic secondary battery, the sealing platecomprising: a pair of mouths for attaching a positive electrode terminaland a negative electrode terminal, one mouth being formed on one endside in a longitudinal direction of the sealing plate, and the othermouth being formed on the other end side; and a gas release valve and anelectrolyte pour hole provided between the pair of mouths, the sealingplate including a front face having a concaved flat face, and anidentification code formed on the concaved flat face.
 2. The sealingplate for a prismatic secondary battery according to claim 1, whereinthe sealing plate includes a back face having a convex portion at aposition corresponding to the concaved flat face.
 3. The sealing platefor a prismatic secondary battery according to claim 2, wherein anadditional identification code is applied on at least one side adjacentto the convex portion in the longitudinal direction of the sealingplate.
 4. The sealing plate for a prismatic secondary battery accordingto claim 3, wherein the convex portion is provided at a positionadjacent to the gas release valve and the additional identification codeis formed at a position opposite to the gas release valve with respectto the convex portion.
 5. A prismatic secondary battery comprising: aprismatic hollow outer body having a mouth and a bottom; a flatelectrode assembly including a positive electrode sheet and a negativeelectrode sheet, a positive electrode collector electrically connectedto the positive electrode sheet, a negative electrode collectorelectrically connected to the negative electrode sheet, and anelectrolyte, all being stored in the prismatic outer body; a sealingplate sealing up the mouth of the prismatic outer body; and a positiveelectrode terminal and a negative electrode terminal attached to thesealing plate in a manner electrically insulated from the sealing plate,while electrically connected to the positive electrode collector and thenegative electrode collector, respectively, on the sealing plate, thepositive electrode terminal being fixed on one end side in alongitudinal direction of the sealing plate and the negative electrodeterminal being fixed on the other end side, a gas release valve and anelectrolyte pour hole being formed between the positive electrodeterminal and the negative electrode terminal, the sealing plateincluding a front face having a concaved flat face, and anidentification code formed on the concaved flat face.